In recent years, the introduction of a radio train control system that abolishes signals besides a railroad track and performs train control using radio is in progress. The system transmits information directly related to safe operation of trains such as speed and stop positions. Therefore, the system requires a radio system to have extremely highly reliable communication performance. In particular, when the system is established using an ISM (Industry Science Medical) band because, for example, an exclusive frequency cannot be secured for train control, it is likely that the radio transmission is interfered by an uncontrollable interference signal. It is necessary to realize a function for surely delivering information even under the presence of a large interference signal.
As a communication system excellent in interference resistance, a system using a DSSS (Direct Sequence Spread Spectrum) is proposed (e.g., Non Patent Literature 1). When a plurality of frequency channels can be used, an interference avoidance method by an FHSS (Frequency Hopping Spread Spectrum) is also proposed (e.g., Non Patent Literature 2). In particular, a communication system using the FHSS has high interference avoidance performance and is widely used in communication systems required to have high reliability. For example, Patent Literature 1 discloses an FHSS system that temporally divides a frame used for transmission into a non-collision access portion controlled by a base station and a random access portion, where collision occurs, to realize efficient communication. Patent Literature 2 discloses a method for improving reliability through “successive transmission” for transmitting data same as the FHSS a plurality of times.